Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture in which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test, also referred to as the unit under test or "UUT." Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in the board must be customized for that particular circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture superimposed on the array of test probes. During testing, the spring-loaded probes are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a "wired test fixture" in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made by using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
The wire-wrapping or other connection of test probes, interface pins and transfer pins for use in a wired test fixture can be time intensive. However, customized wired test fixtures are particularly useful in testing circuit boards with complex arrangements of test points and low-volume production boards where the larger and more complex and expensive electronic test analyzers are not practical.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting signals from the fixture to the external circuit tester. A further class of test fixtures is the so-called "dedicated" test fixtures, also known as a "grid-type fixture," in which the random pattern of test points on the board are contacted by translator pins which transfer test signals to interface pins arranged in a grid pattern in a receiver. In these grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures.
A typical dedicated or grid fixture contains test electronics with a huge number of switches connecting test probes in a grid base to corresponding test circuits in the electronic test analyzer. In one embodiment of a grid tester as many as 40,000 switches are used. When testing a bare board on such a tester, a translator fixture supports translator pins that communicate between a grid pattern of test probes in a grid base and an off-grid pattern of test points on the board under test. In one prior art grid fixture so-called "tilt pins"" are used as the translator pins. The tilt pins are straight solid pins mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture. The tilt pins can tilt in various three-dimensional orientations to translate separate test signals from the off-grid random pattern of test points on the board to the grid pattern of test probes in the grid base.
In the past, translator fixtures have been constructed and assembled with a plurality of translator plates made from a plastic material such as Lexan. The translator plates are stacked in the fixture between corresponding sets of spacers aligned with one another vertically to form "stand-offs" spaced apart around the periphery of the fixture. The spacers hold the translator plates in a fixed position spaced apart vertically from one another and reasonably parallel to each other. The translator plates at each level of the fixture have pre-drilled patterns of alignment holes that control the position of each tilt pin in the translator fixture.
Typically, the grid tester will include a bottom translator fixture positioned on a grid base and a second or top translator fixture positioned on a separate grid base located above the bottom fixture. The top fixture is inverted above the bottom fixture so that a printed circuit board under test is sandwiched between the top plates of the top and bottom fixtures. The top translator fixture includes the same components as the bottom fixture so that test points on the top surface of the printed circuit board are tested by the top fixture and the test points on the bottom surface of the printed circuit board are simultaneously tested by the bottom fixture. Both sets of test data are interpreted by a single electronic test analyzer.
A problem exists in this arrangement when a density imbalance of test pins occurs on one side of the printed circuit board due to the locations of the test pads. For example, the top surface of the particular circuit board under test may not have any test points in the center of the circuit board. Test pins are contacting test pads on the board under pressure in the center of the lower surface, and due to the lack of counteracting force from the top fixture, the printed circuit board is bent upwardly during testing, which can damage the unit under test. Each translator pin applies a force of approximately 6 to 8 ounces which when multiplied by the number of pins concentrated in a small area of the unit under test can create an unopposed force up to as much as 500 pounds.
To resolve this problem, stand-offs have been incorporated in the center section of the fixture corresponding to the area where no test points are contacting test pads. This effort has proven unsuccessful, however, resulting in not only continued bending of the printed circuit board, but the entire top fixture bending, resulting in damage to both the circuit board and the test fixture. Accurate test data are difficult to obtain when the unit under test and/or the test fixture is bending.
Consequently, a need exists for a test fixture design which provides a counteracting force to the unit under test in locations on the board where a significant number of test pads are located on only one surface.